Conventionally, there is a known uninterruptible power supply system which drives a load by a plurality of uninterruptible power supply devices connected in parallel. In the conventional first uninterruptible power supply system, a load current is equally shared among all of the uninterruptible power supply devices to drive a load. In this system, even when a failure occurs in one uninterruptible power supply device, the load can be driven by the remaining uninterruptible power supply devices.
Furthermore, for example, as disclosed in Japanese Patent Laying-Open No. 03-124228 (Patent Literature 1) and Japanese Utility Model Laying-Open No. 04-51044 (Patent Literature 2), in the conventional second uninterruptible power supply system, the number of uninterruptible power supply devices to be operated is changed in accordance with the load current.